The present invention relates to circulating fluidized bed boilers and pertains particularly to a system and method for controlling reheat steam temperature.
Several methods are presently known for controlling reheat steam temperatures. One method of reheater temperature control is the use of a system for gas bypass over the reheater. Two separate flue gas passes are provided in the convection pass of the boiler (one for superheater and one for reheater), with means such as dampers downstream of each to vary the amount of flue gas flow over each section. The outlet steam temperature of the reheater can be controlled by varying the amount of flue gas flow between the convection pass sections. The main disadvantage of this system is that the dampers are located in a higher temperature (500-700 degrees F.), dust laden flue gas path making them susceptible to erosion and mechanical failure. Also, the steam temperature control range is limited with this type of system.
Another method of reheater outlet steam temperature control is by the use of external heat exchangers. With this approach, a portion of the recirculated solids within the circulating fluidized bed system is diverted to an externally mounted fluidized bed heat exchanger, i.e. external heater exchanger (EHE), in which a section of or complete reheater is located. By varying the amount of solids flow to the EHE, the quantity of heat transfer to the reheater and the reheater outlet steam temperature is controlled. The main disadvantages of this system are that the solids flow control valve is a high maintenance item, and the reheat tube surface within the EHE is subject to erosion. This effects the availability of the unit.
A further approach to the control of the reheater outlet steam temperature is by the use of spray desuperheater. This approach utilizes spraying water for desuperheating, and thereby controlling reheater outlet steam temperature. This is a simple approach, but not generally accepted, because it degrades the cycle efficiency.
Still another approach is by the use of excess air. Excess air supplied to the boiler can be used for reheat steam temperature control. This approach, however, is not favored because of its negative affect on boiler efficiency.
A still further approach is by the use of gas recirculation. By this approach, large quantities of flue gases are recirculated to achieve the rated reheater outlet steam temperature. This approach, however, requires the use of a gas recirculation fan for handling a hot dust laden gas and requires additional power consumption, which makes this approach disadvantageous.
In our prior application, we disclose and claim a steam generator having a fluidized bed combustion system that includes a fluidized bed combustor, at least one separator, and a reheater in a flue gas pass having at least first and second or final stages (possibly more), and includes means for controlling the temperature of the second or final stage of the reheater comprising of a means for bypassing a selected portion of cold steam around said first stage reheater directly to said second or final stage reheater. We have discovered a rearrangement of the superheater and reheater that enables the reheater to pick up more heat at lower loads and potentially extend its steam temperature control range. This system will also enhance the coupling of two units to one turbine easier as to temperature matching capabilities.
Accordingly, the present invention is directed to an improved method and system for reheat steam temperature control.